Field of the Invention
This invention relates to a generator of short optical pulses having a very high temporal contrast.
Description of the Related Art
Generally, it is known that pulsed laser sources are characterised by a temporal confinement of the light emission. This confinement makes it possible to carry out laser-material interactions, in conditions that combine a very short duration and a very high instant power for a given energy of the laser pulse. The first characteristic of this pulse is its duration such as defined for example by the temporal separation between the two instants when the emitted power is half the maximum power (duration at mid-height). This characteristic is not however sufficient for defining the pertinent properties of the source. Indeed, the existence of low power levels, such as for example the thousands of the maximum power, at instants offset from the instant of the maximum by a delay that is significantly greater than the duration at mid-height (for example a few picoseconds), are able to profoundly disturb the laser-material interaction. By way of example, the interaction of a pulse with a high power density with a solid target will be fully disturbed if the central pulse is preceded by parasite power levels that exceed the ionisation limit of the atmosphere surrounding the solid target. This limit can be much less than the maximum power.
We are therefore led to characterise an ultrafast pulse source by its temporal contrast. The notion of temporal contrast designates the ratio of the intensity between the maximum of the illuminance at a point (expressed in W or W/cm2) and the illuminance at this same point but at different instants. The contrast is characterised by different values at different delays with respect to the main pulse. For example: 104 at −3 ps and 1014 at −100 ps.
By way of example of mechanisms that can produce parasite power levels that degrade the contrast, the following can be mentioned:                spontaneous emission and amplified spontaneous emission, present in all laser amplifiers,        parametric fluorescence, present in all optical parametric amplifiers,        the responses of the main pulse caused by the reflections on the optical surfaces of the transparent mediums passed through,        the parasite or residual pulses when the main pulse is taken from a pulse train,        certain amplitude or spectral phase defects induced by the amplification system, the beam transport system, the dielectric treatment (thin reflective or anti-reflective layers), or by surface defects of the optics used in spectrally dispersive systems (draft for example),        the non-linear effects that couple and aggravate the aforementioned mechanisms.        